Improvisation is a kind of musical performance, which is a combination of creating music and playing music simultaneously, and the real pleasure resides in expressing the player's own intention and emotion widely and freely. However, playing an improvisation will require a high level of musical knowledge and music playing skill, and will be a very hard performance for a player (user) having insufficient knowledge and little experience. It would, therefore, be helpful for those players, if some assisting technology should be developed and presented with which even an unskilled player could express his/her own intention and emotion to his/her content. An example of an apparatus for assisting in performing an improvisation in the art is shown in unexamined Japanese patent publication No. 2004-206073, in which the notes input from the key depression by the player are changed to the chord notes or the tension notes in real time to be output as tones by means of the tone generator. With such an apparatus, however, the produced notes are limited within the harmonization voicing by the basic chord notes and the tension notes, even if whichever keys the player depresses. And accordingly, a highly skilled performance suited for an improvisation would not be realized.